Electrochemical reaction device

ABSTRACT

An electrochemical reaction device includes: an electrode layer including a cathode, an ion exchange membrane, and an anode that are stacked in this order; a first flow path structure including an anode-specific flow path as a passage for an electrolytic solution defined by a surface on one side and a cathode-specific flow path as a passage for an electrolytic solution containing dissolved carbon dioxide defined by a surface on the other side; and a second flow path structure including an anode-specific flow path as a passage for an electrolytic solution defined by a surface on one side and a cathode-specific flow path as a passage for an electrolytic solution containing dissolved carbon dioxide defined by a surface on the other side. The first flow path structure, the electrode layer, the second flow path structure, and the electrode layer are stacked in this order repeatedly to form an electrolytic cell stacking structure.

This application is based on and claims the benefit of priority from Japanese Patent Application No. 2022-039442, filed on 14 Mar. 2022, the content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an electrochemical reaction device that reduces carbon dioxide.

Related Art

A technology to capture exhaust gas and carbon dioxide in the atmosphere and electrochemically reduce the captured exhaust gas and carbon dioxide to obtain valuable resources is promising as it has a possibility of achieving carbon neutrality.

According to a technology known as this type of technology to capture carbon dioxide, carbon dioxide in gas is physically or chemically adsorbed on a solid or liquid adsorbent and then desorbed by energy such as heat for use. According to a technology known as a technology to electrochemically reduce carbon dioxide, a cathode having a catalyst layer formed using a carbon dioxide reduction catalyst on a side of a gas diffusion layer in contact with an electrolytic solution receives carbon dioxide gas supplied from a side opposite to the catalyst layer on the gas diffusion layer, and the carbon dioxide is electrochemically reduced (see, for example, PCT International Publication No. WO2018/232515).

-   Patent Document 1: PCT International Publication No. WO2018/232515

SUMMARY OF THE INVENTION

Examples of a method of supplying carbon dioxide to an electrochemical reaction device to reduce carbon dioxide include a method of supplying carbon dioxide in a gas form and a method of dissolving carbon dioxide in an electrolytic solution and supplying the carbon dioxide in a liquid form as carbonate ions. While improving economic efficiency is an issue to be solved for the purpose of achieving carbon neutrality, the method of supplying carbon dioxide in a liquid form works advantageously in terms of energy efficiency for reasons that it does not require input energy that is generally required in a desorbing step in a device for capturing carbon dioxide, it is unlikely to be subjected to efficiency impact as oxygen does not dissolves in an electrolytic solution even at low purity of carbon dioxide, it consumes a small amount of energy after electrolysis as generated gas is easily separated from a raw material through gas-liquid separation, etc. However, this technology has not been put to industrialization under present circumstances. Hence, it has still room for improvement in terms of improving energy efficiency while imparting alkali resistance to a strong alkaline electrolytic solution to an electrolytic cell responsible for reducing treatment.

The present invention is intended to provide an electrochemical reaction device that is capable of efficiently performing a treatment of reducing carbon dioxide supplied while dissolved in an electrolytic solution.

(1) The present invention relates to an electrochemical reaction device (electrochemical reaction device 3 described later, for example) for reducing carbon dioxide. The electrochemical reaction device includes: an electrode layer (electrode layer 30 described later, for example) including a cathode (cathode 21 described later, for example), an ion exchange membrane (ion exchange membrane 23 described later, for example), and an anode (anode 22 described later, for example) that are stacked in this order; a first flow path structure (first flow path structure 31 described later, for example) including an anode-specific flow path (anode-specific flow path 41 described later, for example) as a passage for an electrolytic solution defined by a surface on one side and a cathode-specific flow path (cathode-specific flow path 43 described later, for example) as a passage for a cathode-specific electrolytic solution containing dissolved carbon dioxide defined by a surface on an other side; and a second flow path structure (second flow path structure 32 described later, for example) including an anode-specific flow path (anode-specific flow path 51 described later, for example) as a passage for an anode-specific electrolytic solution defined by a surface on one side and a cathode-specific flow path (cathode-specific flow path 53 described later, for example) as a passage for a cathode-specific electrolytic solution containing dissolved carbon dioxide defined by a surface on an other side. The first flow path structure, the electrode layer, the second flow path structure, and the electrode layer are stacked in this order repeatedly to form an electrolytic cell stacking structure (electrolytic cell stacking structure 13 described later, for example).

As a result, each of the first flow path structure and the second flow path structure defines the cathode-specific flow path and the anode-specific flow path responsible for different electrode layers. This achieves compact size of the electrolytic cell stacking structure in a stacking direction, making it possible to provide a large number of the electrode layers in limited space. Thus, the treatment of reducing carbon dioxide can be performed efficiently.

(2) In the electrochemical reaction device described in (1), the electrolytic cell stacking structure may include: a cathode-specific electrolytic solution inlet (cathode-specific electrolytic solution inlet 61 described later, for example) through which a cathode-specific electrolytic solution is supplied; a cathode-specific electrolytic solution outlet (cathode-specific electrolytic solution outlet 62 described later, for example) through which the cathode-specific electrolytic solution after reaction is discharged and which is arranged on the opposite side to the cathode-specific electrolytic solution inlet across the first flow path structure and the second flow path structure; an anode-specific electrolytic solution inlet (anode-specific electrolytic solution inlet 63 described later, for example) through which an anode-specific electrolytic solution is supplied; and an anode-specific electrolytic solution outlet (anode-specific electrolytic solution outlet 64 described later, for example) through which the anode-specific electrolytic solution after reaction is discharged which is arranged on the opposite side to the anode-specific electrolytic solution inlet across the first flow path structure and the second flow path structure. The cathode-specific electrolytic solution inlet, the cathode-specific electrolytic solution outlet, the anode-specific electrolytic solution inlet, and the anode-specific electrolytic solution outlet may be arranged in such a manner that a line connecting the cathode-specific electrolytic solution inlet to the cathode-specific electrolytic solution outlet and a line connecting the anode-specific electrolytic solution inlet to the anode-specific electrolytic solution outlet intersect each other as viewed in a stacking direction of the electrolytic cell stacking structure.

As a result, the electrolytic solutions are to flow in such a manner as to intersect each other as viewed in the stacking direction. Generally, if two or more raw materials are prepared and an approximately uniform reaction speed is intended in designing a reactor, countercurrent flow is employed in many cases. Meanwhile, employing the above-described configuration realizes cross flow for causing the electrolytic solutions to flow in nearly countercurrent flow. This simulates the concept of the countercurrent flow to achieve a more stabilized reaction speed.

(3) In the electrochemical reaction device described in (1) or (2), in the electrolytic cell stacking structure, the cathode-specific electrolytic solution inlet (cathode-specific electrolytic solution inlet 61 described later, for example) through which a cathode-specific electrolytic solution is supplied and the anode-specific electrolytic solution inlet (anode-specific electrolytic solution inlet 63 described later, for example) through which an anode-specific electrolytic solution is supplied may be arranged below the first flow path structure and the second flow path structure, and the cathode-specific electrolytic solution outlet (cathode-specific electrolytic solution outlet 62 described later, for example) through which the cathode-specific electrolytic solution after reaction is discharged and the anode-specific electrolytic solution outlet (anode-specific electrolytic solution outlet 64 described later, for example) through which the anode-specific electrolytic solution after reaction is discharged may be arranged above the first flow path structure and the second flow path structure.

If gas generated at each of the cathode and the anode stays in the cathode or the anode for failing to be discharged smoothly, an area of contact between an electrolytic solution and the electrode layer is reduced to cause the risk of adverse effect on a reaction speed. However, employing the above-described configuration achieves smooth discharge of the generated gas using buoyant force.

(4) In the electrochemical reaction device described in any one of (1) to (3), the electrolytic cell stacking structure may include an insulating part (insulating part 60 described later, for example) having elasticity and surrounding a periphery of each of the electrode layer, the first flow path structure, and the second flow path structure.

As a result, the insulating part can be used for adjusting force of interposing the ion exchange membrane with the cathode and the anode. The insulating part can also be used for preventing an electrolytic solution from flowing out of the electrolytic cell stacking structure.

(5) In the electrochemical reaction device described in any one of (1) to (4), the first flow path structure may include a plurality of convex anode-side contact parts (anode-side contact parts 42 described later, for example) formed on the surface defining the anode-specific flow path and a plurality of convex cathode-side contact parts (cathode-side contact parts 44 described later, for example) formed on the surface defining the cathode-specific flow path, the second flow path structure may include a plurality of convex anode-side contact parts (anode-side contact parts 52 described later, for example) formed on the surface defining the anode-specific flow path and a plurality of convex cathode-side contact parts (cathode-side contact parts 54 described later, for example) formed on the surface defining the cathode-specific flow path, the anode-side contact parts of the first flow path structure and the cathode-side contact parts of the second flow path structure may be arranged at positions corresponding to each other in a stacking direction of the electrolytic cell stacking structure, and the cathode-side contact parts of the first flow path structure and the anode-side contact parts of the second flow path structure may be arranged at positions corresponding to each other in the stacking direction of the electrolytic cell stacking structure.

As a result, it is possible to adjust force of interposing the ion exchange membrane with the cathode and the anode using the heights of the anode-side contact parts and those of the cathode-side contact parts. This allows the thickness of the electrode layer to be physically controlled with higher accuracy, so that lack of balance in electrical resistance due to thickness differences can be reduced.

(6) In the electrochemical reaction device described in (5), in each of the first flow path structure and the second flow path structure, the anode-side contact parts may be arranged in a plurality of rows aligned in a direction intersecting a flow direction of an electrolytic solution, the cathode-side contact parts may be arranged in a plurality of rows aligned in a direction intersecting a flow direction of an electrolytic solution, and the anode-side contact parts and the cathode-side contact parts may be arranged alternately in such a manner as to avoid overlap therebetween as viewed in the stacking direction of the electrolytic cell stacking structure.

As a result, by the presence of the rows of the cathode-side contact parts or those of the anode-side contact parts, an electrolytic solution is allowed to develop separately to the right and left several times in a flow direction of the electrolytic solution and to flow uniformly, thereby contributing to uniformity in current density on a reaction surface.

(7) In the electrochemical reaction device described in any one of (1) to (6), the first flow path structure may have a shape point-symmetrical to the second flow path structure and may become identical in shape to the second flow path structure by being rotated 180 degrees as viewed in a stacking direction.

This allows reduction in parts count required for the electrolytic cell stacking structure, so that manufacturing cost can be reduced.

(8) In the electrochemical reaction device described in any one of (1) to (7), the cathode-specific flow path and the anode-specific flow path may each include a flow path guide wall (flow path guide walls 250 to 253, 260 to 263 described later, for example) for guiding a flow of an electrolytic solution.

As a result, it is possible to form a flow path configured to allow an electrolytic solution to flow sufficiently even to a distant position to provide a more uniform flow of the electrolytic solution, thereby contributing to uniformity in current density on a reaction surface.

According to the present invention, it is possible to provide an electrochemical reaction device that allows a treatment of reducing supplied carbon dioxide dissolved in an electrolytic solution to be performed efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the configuration of a carbon dioxide treatment apparatus including an electrochemical reaction device according to an embodiment of the present invention;

FIG. 2 is a perspective view showing the configuration of an electrolytic cell stacking structure of the electrochemical reaction device according to the present embodiment;

FIG. 3 is a sectional view along a line A-A in FIG. 2 showing the interior of the electrolytic cell stacking structure of the electrochemical reaction device according to the present embodiment;

FIG. 4 is a sectional view along a line B-B in FIG. 2 showing the interior of the electrolytic cell stacking structure of the electrochemical reaction device according to the present embodiment;

FIG. 5 is a view showing the electrolytic cell stacking structure of the electrochemical reaction device according to the present embodiment, as viewed from one side in a stacking direction;

FIG. 6 is a view showing an electrolytic cell stacking structure of an electrochemical reaction device according to a first modification, as viewed from one side in a stacking direction;

FIG. 7 is a view showing an electrolytic cell stacking structure of an electrochemical reaction device according to a second modification, as viewed from one side in a stacking direction; and

FIG. 8 is a view showing a cathode-side surface and an anode-side surface of a first flow path structure according to the second modification.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention will be described below by referring to the drawings.

FIG. 1 is a schematic view showing the configuration of a carbon dioxide treatment apparatus 1 including an electrochemical reaction device 3 according to an embodiment of the present invention. As shown in FIG. 1 , the carbon dioxide treatment apparatus 1 according to an embodiment of the present invention includes a capturing device 2, the electrochemical reaction device 3, and a homologation reaction device 4 as principal structures.

The capturing device 2 captures carbon dioxide from a capture target supplied from outside that may be the atmosphere or exhaust gas, for example. The capturing device 2 includes a concentration unit 11 and an absorption unit 12.

The concentration unit 11 performs a treatment of concentrating a captured target such as the atmosphere or exhaust gas. The concentration unit 11 is configured using a membrane separation device or an adsorption/desorption device utilizing chemical or physical adsorption and desorption, for example. The concentrated gas concentrated by the concentration unit 11 is fed to the absorption unit 12.

The absorption unit 12 performs a treatment of bringing carbon dioxide gas in the concentrated gas supplied from the concentration unit 11 into contact with an electrolytic solution and dissolving carbon dioxide in the electrolytic solution. A method of bringing the carbon dioxide gas and the electrolytic solution into contact with each other is not particularly limited but can be a method of blowing the concentrated gas into the electrolytic solution and performing bubbling.

In the absorption unit 12, an electrolytic solution such as KOH composed of a strong alkaline aqueous solution is used as an absorption solution for absorbing carbon dioxide, for example. In carbon dioxide, a carbon atom is positively charged (δ+) because oxygen atoms strongly attract electrons. Thus, in a strong alkaline aqueous solution where hydroxide ions are present in large quantity, the dissolution reaction of carbon dioxide easily proceeds from a hydrated state to CO₃ ²⁻ via HCO₃ ⁻ to produce an equilibrium state where the abundance of CO₃ ²⁻ is high. This shows that carbon dioxide dissolves more readily in a strong alkaline aqueous solution than other gases such as nitrogen, hydrogen, and oxygen, and carbon dioxide in the concentrated gas is selectively absorbed in the electrolytic solution in the absorption unit 12. Using the electrolytic solution in the absorption unit 12 in this way makes it possible to assist in the concentration of carbon dioxide. This eliminates the need of concentrating carbon dioxide to a high concentration in the concentration unit 11, thereby allowing reduction in energy required for concentration in the concentration unit 11. The electrolytic solution containing the carbon dioxide absorbed in the absorption unit 12 is fed to the electrochemical reaction device 3. An electrolytic solution having flowed out of the electrochemical reaction device 3 may be circulated to become available as the electrolytic solution to be used in the absorption unit 12.

The electrochemical reaction device 3 is a device that electrochemically reduces carbon dioxide. The electrochemical reaction device 3 reduces carbon dioxide using an electrolytic cell stacking structure 13 formed by stacking electrolytic cells on each other for reducing carbon dioxide. The configuration of the electrolytic cell stacking structure 13 will be described later in detail. Ethylene generated through reduction of carbon dioxide using the electrolytic cell stacking structure 13 of the electrochemical reaction device 3 is fed to the homologation reaction device 4.

The homologation reaction device 4 is a device that increases the number of carbons by multimerizing ethylene generated through reduction of carbon dioxide in the electrochemical reaction device 3. The homologation reaction device 4 includes a reactor 14 and a gas-liquid separator 15.

In the reactor 14, the multimerization reaction of ethylene is carried out in the presence of an olefin multimerization catalyst, for example, thereby producing olefins having an extended carbon chain such as 1-butene, 1-hexene, and 1-octene. The olefin multimerization catalyst is a solid acid catalyst using silica alumina or zeolite as a carrier or a transition metal complex compound, for example.

The gas-liquid separator 15 performs gas-liquid separation on generated gas resulting from the multimerization reaction in the reactor 14. An olefin having 6 or more carbon atoms is a liquid at room temperature. Thus, if an olefin having 6 or more carbon atoms is a target carbon compound, setting a temperature at the gas-liquid separator 15 about 30° C. makes it possible to easily make gas-liquid separation into an olefin having 6 or more carbon atoms and an olefin having less than 6 carbon atoms. Furthermore, increasing a temperature at the gas-liquid separator 15 makes it possible to increase a carbon number in a resultant olefin liquid.

While the configuration of the carbon dioxide treatment apparatus 1 has briefly been described above, the configuration of the carbon dioxide treatment apparatus 1 is not limited to the above-described configuration. In another configuration, an electrolytic solution used in the electrochemical reaction device 3 may also be used as an absorption solution in the absorption unit 12 of the capturing device 2, and carbon dioxide may be supplied to the electrochemical reaction device 3 while dissolving in the electrolytic solution and may be electrochemically reduced, for example. This reduces energy required for desorption of carbon dioxide, compared to a case where carbon dioxide is adsorbed on an adsorbent, desorbed by heating and then reduced. As a result, energy efficiency can be increased and carbon dioxide loss can be reduced.

An exemplary configuration of the electrolytic cell stacking structure 13 will be described next. FIG. 2 is a perspective view showing the configuration of the electrolytic cell stacking structure 13 of the electrochemical reaction device 3 according to the present embodiment. FIGS. 3 and 4 are a sectional view along a line A-A in FIG. 2 and a sectional view along a line B-B in FIG. 2 respectively showing the interior of the electrolytic cell stacking structure 13 of the electrochemical reaction device 3 according to the present embodiment. FIG. 5 is a view showing the electrolytic cell stacking structure 13 of the electrochemical reaction device 3 according to the present embodiment, as viewed from one side in a stacking direction.

The electrolytic cell stacking structure 13 includes an electrode layer 30, a first flow path structure 31, a second flow path structure 32, and an insulating part 60.

The electrode layer 30 is an electrolytic cell configured by interposing an ion exchange membrane 23 with a cathode 21 and an anode 22.

The cathode 21 is an electrode that reduces carbon dioxide electrochemically to generate carbon compounds and reduces water to generate hydrogen. The cathode 21 is configured using a gas diffusion layer and a cathode catalyst layer, for example.

The gas diffusion layer may be any layer as long as it allows generated gaseous carbon compounds and hydrogen to permeate therethrough. The gas diffusion layer of the cathode 21 is not particularly limited, and examples thereof include carbon paper and carbon cloth.

As a cathode catalyst forming the cathode catalyst layer, a publicly-known catalyst that promotes reduction of carbon dioxide is usable. Specific examples of the cathode catalyst include: metals such as gold, silver, copper, platinum, palladium, nickel, cobalt, iron, manganese, titanium, cadmium, zinc, indium, gallium, lead, and tin; alloys and intermetallic compounds of these metals; and metal complexes such as a ruthenium complex and a rhenium complex. Among these, copper and silver are preferable, and copper is more preferable from the viewpoint of facilitating reduction of carbon dioxide. As the cathode catalyst, one type may be used alone, or two or more types may be used in combination. As the cathode catalyst, a supported catalyst where metal particles are supported on a carbon material (carbon particles, carbon nanotubes, graphene, and the like) is usable.

The ion exchange membrane 23 is arranged between the cathode 21 and the anode 22. The ion exchange membrane 23 is an anionic exchange membrane (AEM) that transfers OH⁻ generated at the cathode 21 toward the anode 22.

The anode 22 is an electrode for oxidizing hydroxide ions to generate oxygen. The anode 22 is configured using a gas diffusion layer and an anode catalyst layer, for example.

The gas diffusion layer may be any layer as long as it allows generated oxygen to permeate therethrough. Examples of the gas diffusion layer of the anode 22 include carbon paper and carbon cloth. A porous body such as a mesh material, a punching material, a porous material, or a metal fiber sintered body is also usable as this gas diffusion layer. Examples of a material for the porous body include metals such as titanium, nickel, and iron, and alloys (for example, SUS) of these metals.

The anode catalyst layer is not particularly limited but a publicly-known anode catalyst is usable. Specific examples thereof include: metals such as platinum, palladium, and nickel; alloys and intermetallic compounds of these metals; metal oxides such as manganese oxide, iridium oxide, nickel oxide, cobalt oxide, iron oxide, tin oxide, indium oxide, ruthenium oxide, lithium oxide, and lanthanum oxide; and metal complexes such as a ruthenium complex and a rhenium complex. As the anode catalyst, one type may be used alone, or two or more types may be used in combination.

The first flow path structure 31 will be described next. The first flow path structure 31 is formed into a plate-like shape using metal having conductivity such as Ni-plated SUS, for example. The first flow path structure 31 includes a planar portion on one side in the stacking direction, and the planar portion defines an anode-specific flow path 41 and has a convex anode-side contact part 42 formed thereon. The first flow path structure 31 includes a planar portion on the other side in the stacking direction, and the planar portion defines a cathode-specific flow path 43 and has a convex cathode-side contact part 44 formed thereon.

In the present embodiment, the first flow path structure 31 is formed by press forming. The cathode-side contact part 44 has a concave shape as viewed from the side of the anode 22 and the anode-side contact part 42 has a concave shape as viewed from the side of the cathode 21. The anode-side contact part 42 and the cathode-side contact part 44 are arranged alternately.

With focus on the arrangement of the anode-side contact part 42, a plurality of anode-side contact parts 42 are arranged in a direction intersecting a flow direction in the anode-specific flow path 41 and separated from each other in consideration of the cathode-side contact part 44. The anode-side contact parts 42 are arranged in a plurality of rows formed in the flow direction. Likewise, with focus on the cathode-side contact part 44, a plurality of cathode-side contact parts 44 are arranged in a direction intersecting a flow direction in the cathode-specific flow path 43 and separated from each other in consideration of the anode-side contact part 42. The cathode-side contact parts 44 are arranged in a plurality of rows formed in the flow direction. The anode-side contact part 42 and the cathode-side contact part 44 are arranged at positions not overlapping each other in the stacking direction.

The second flow path structure 32 will be described next. The second flow path structure 32 is formed into a plate-like shape using metal having conductivity such as Ni-plated SUS, for example. The second flow path structure 32 includes a planar portion on one side in the stacking direction, and the planar portion defines an anode-specific flow path 51 and has a convex anode-side contact part 52 formed thereon. The second flow path structure 32 includes a planar portion on the other side in the stacking direction and the planar portion defines a cathode-specific flow path 53 and has a convex cathode-side contact part 54 formed thereon.

In the present embodiment, the second flow path structure 32 is formed by press forming. The anode-side contact part 52 has a concave shape as viewed from the side of the cathode 21 and the cathode-side contact part 54 has a concave shape as viewed from the side of the anode 22. The cathode-side contact part 54 and the anode-side contact part 52 are arranged alternately.

With focus on the arrangement of the cathode-side contact part 54, a plurality of cathode-side contact parts 54 are arranged in a direction intersecting a flow direction in the cathode-specific flow path 53 and separated from each other in consideration of the anode-side contact part 52. The cathode-side contact parts 54 are arranged in a plurality of rows formed in the flow direction. Likewise, with focus on the anode-side contact part 52, a plurality of anode-side contact parts 52 are arranged in a direction intersecting a flow direction in the anode-specific flow path 51 and separated from each other in consideration of the cathode-side contact part 54. The anode-side contact parts 52 are arranged in a plurality of rows formed in the flow direction. Namely, the cathode-side contact part 54 and the anode-side contact part 52 are arranged at positions not overlapping each other in the stacking direction.

The cathode-side contact part 44 of the first flow path structure 31 and the anode-side contact part 52 of the second flow path structure 32 are arranged at positions overlapping each other in the stacking direction. The cathode-side contact part 44 presses the cathode 21 against the ion exchange membrane 23, and the anode-side contact part 52 presses the anode 22 against the ion exchange membrane 23.

This configuration is also defined in the electrode layer 30 adjacent in the stacking direction. The cathode-side contact part 54 of the second flow path structure 32 and the anode-side contact part 42 of the first flow path structure 31 are arranged at positions overlapping each other in the stacking direction. The cathode-side contact part 54 presses the cathode 21 against the ion exchange membrane 23, and the anode-side contact part 42 presses the anode 22 against the ion exchange membrane 23.

The insulating part 60 will be described next. The insulating part 60 is a frame-like part arranged for each of the first flow path structure 31, the electrode layer 30, and the second flow path structure 32. FIG. 4 shows the insulating part 60 holding the first flow path structure 31 as an insulating part 60 a, the insulating part 60 holding the electrode layer 30 as an insulating part 60 b, and the insulating part 60 holding the second flow path structure 32 as an insulating part 60 c. Explanation common between the insulating parts 60 will be given by assigning signs without alphabets.

The insulating part 60 a includes a groove 65 a where the first flow path structure 31 is held. The insulating part 60 b includes a groove 65 b where the electrode layer 30 is held. The insulating part 60 c includes a groove 65 c where the second flow path structure 32 is held. The insulating part 60 is made of an elastic material having alkali resistance such as ethylene propylene diene methylene linkage (EPDM), and functions as a sealing member for preventing an electrolytic solution from flowing out of the electrolytic cell stacking structure 13.

The electrode layer 30 is stored while surface pressure is applied to the first flow path structure 31 and the second flow path structure 32 using the insulating part 60. This provides a configuration where the cathode 21 and the anode 22 are physically forced into the ion exchange membrane 23 using the first flow path structure 31 and the second flow path structure 32.

The ion exchange membrane 23 used under alkaline conditions is not a proton exchange membrane (PEM) but an AEM. In the case of the PEM, electrical resistance is minimized as a result of an established technology for forming a membrane-electrode assembly where an ion exchange membrane is chemically connected to the cathode 21 and the anode 22. Meanwhile, in the case of the AEM, electrical resistance is preferably minimized by pressing the cathode 21 and the anode 22 against the ion exchange membrane 23 and minimizing distances therebetween using physical force, like in the present embodiment.

Controlling the thickness of the electrode layer 30 using physical force allows adjustment of characteristics. For example, by setting the thickness of the electrode layer 30 equal to or greater than 1.0 mm, different electrical resistance environments are defined in a thickness direction to generate a tendency toward higher electrical resistance particularly on an outer side. This increases loss as a whole to increase an overvoltage. However, a volume of reaction per unit area is increased to produce an advantage in terms of current density. Conversely, setting the thickness of the electrode layer 30 equal to or less than 0.5 mm generates a uniform electrical resistance environment as a whole to produce an advantage in terms of voltage. However, this is disadvantageous in terms of current density. Thus, to increase current density, it is preferable to increase the quantity of a supported active substance or increase the absolute value of current by increasing the number of stacks.

In the present embodiment, the thickness of the electrode layer 30 is set equal to or less than 0.5 mm in consideration of electrical resistance. In this configuration, if each of the cathode 21 and the anode 22 is set at a thickness of 0.3 mm, for example, layout described next is defined. By setting the thickness of the insulating part 60 as a rubber part and setting the depths of press of the first flow path structure 31 and the second flow path structure 32, a thickness of 0.5 mm is ensured for each of the cathode-specific flow path and the anode-specific flow path. This provides a unit forming the electrode layer 30 with a thickness of 2.1 mm. As a result, it becomes possible to achieve a compact size of the electrolytic cell stacking structure 13 in the stacking direction.

The insulating part 60 includes a cathode-specific electrolytic solution inlet 61 through which an electrolytic solution is introduced into the cathode 21, a cathode-specific electrolytic solution outlet 62 through which the electrolytic solution after reaction is discharged, an anode-specific electrolytic solution inlet 63 through which an electrolytic solution is introduced into the anode 22, and an anode-specific electrolytic solution outlet 64 through the electrolytic solution after reaction is discharged. The insulating part 60 further includes an inlet passage 71 and an outlet passage 72 for connecting the cathode-specific electrolytic solution inlet 61 and the cathode-specific electrolytic solution outlet 62 to the flow path for the electrolytic solution, or includes an inlet passage 73 and an outlet passage 74 for connecting the anode-specific electrolytic solution inlet 63 and the anode-specific electrolytic solution outlet 64 to a flow path for an electrolytic solution.

As shown in FIG. 5 , the cathode-specific electrolytic solution inlet 61 and the cathode-specific electrolytic solution outlet 62 are arranged at positions shifted from each other in a direction in which the first flow path structure 31 and the second flow path structure 32 are interposed therebetween. Specifically, a line forming connection between the cathode-specific electrolytic solution inlet 61 and the cathode-specific electrolytic solution outlet 62 extends obliquely relative to a direction in which a flow path forming plate is interposed therebetween. Likewise, the anode-specific electrolytic solution inlet 63 and the anode-specific electrolytic solution outlet 64 are arranged at positions shifted from each other in a direction in which the first flow path structure 31 and the second flow path structure 32 are interposed therebetween. Specifically, a line forming connection between the anode-specific electrolytic solution inlet 63 and the anode-specific electrolytic solution outlet 64 extends obliquely relative to a direction in which the flow path forming plate is interposed therebetween. As indicated by arrows in FIG. 5 , the line forming connection between the anode-specific electrolytic solution inlet 63 and the anode-specific electrolytic solution outlet 64 and the line forming connection between the cathode-specific electrolytic solution inlet 61 and the cathode-specific electrolytic solution outlet 62 are configured to intersect each other.

In the electrolytic cell stacking structure 13, by applying a voltage between the cathode 21 and the corresponding anode 22, carbon dioxide is reduced electrochemically to generate carbon compounds and water is reduced to generate hydrogen at the cathode 21. Examples of the carbon compounds generated by reducing carbon dioxide at the cathode 21 include carbon monoxide, ethylene, and ethanol. For example, the following reaction is caused to generate carbon monoxide and ethylene as gaseous products. Hydrogen is also generated at the cathode 21 by the following reaction. The generated gaseous carbon compounds and hydrogen permeate through the gas diffusion layer of the cathode 21 and are then discharged.

CO₂+H₂O→CO+2OH⁻

2CO+8H2O→C₂H₄+8OH⁻+2H₂O

2H₂O→H₂+2OH⁻

Hydroxide ions generated at the cathode 21 move to the anode 22 through the ion exchange membrane 23 and are oxidized by the following reaction to generate oxygen. On the side of the anode 22, the generated oxygen and an electrolytic solution are mixed and discharged to the outside of the cell.

4OH⁻→O₂+2H₂O

As described above, the electrochemical reaction device 3 of the embodiment includes: the electrode layer 30 including the cathode 21, the ion exchange membrane 23, and the anode 22 that are stacked on each other; the first flow path structure 31 including the anode-specific flow path 41 as a passage for an electrolytic solution defined by a surface on one side and the cathode-specific flow path 43 as a passage for a cathode-specific electrolytic solution containing dissolved carbon dioxide defined by a surface on the other side; and the second flow path structure 32 including the anode-specific flow path 51 as a passage for an anode-specific electrolytic solution defined by a surface on one side and the cathode-specific flow path 53 as a passage for a cathode-specific electrolytic solution containing dissolved carbon dioxide defined by a surface on the other side. The first flow path structure 31, the electrode layer 30, the second flow path structure 32, and the electrode layer 30 are stacked in this order repeatedly to form the electrolytic cell stacking structure 13.

As a result, each of the first flow path structure 31 and the second flow path structure 32 defines the cathode-specific flow path and the anode-specific flow path responsible for different electrode layers 30. This achieves compact size of the electrolytic cell stacking structure 13 in the stacking direction, making it possible to provide a large number of the electrode layers 30 in limited space. Thus, the treatment of reducing carbon dioxide can be performed efficiently.

In the present embodiment, the electrolytic cell stacking structure 13 includes: the cathode-specific electrolytic solution inlet 61 through which a cathode-specific electrolytic solution is supplied; the cathode-specific electrolytic solution outlet 62 through which the cathode-specific electrolytic solution after reaction is discharged and which is arranged on the opposite side to the cathode-specific electrolytic solution inlet 61 across the first flow path structure 31 and the second flow path structure 32; the anode-specific electrolytic solution inlet 63 through which an anode-specific electrolytic solution is supplied; and the anode-specific electrolytic solution outlet 64 through which the anode-specific electrolytic solution after reaction is discharged and is arranged on the opposite side to the anode-specific electrolytic solution inlet 63 across the first flow path structure 31 and the second flow path structure 32. The cathode-specific electrolytic solution inlet 61, the cathode-specific electrolytic solution outlet 62, the anode-specific electrolytic solution inlet 63, and the anode-specific electrolytic solution outlet 64 are arranged in such a manner that a line connecting the cathode-specific electrolytic solution inlet 61 to the cathode-specific electrolytic solution outlet 62 and a line connecting the anode-specific electrolytic solution inlet 63 to the anode-specific electrolytic solution outlet 64 intersect each other as viewed in the stacking direction of the electrolytic cell stacking structure 13.

As a result, the electrolytic solutions are to flow in such a manner as to intersect each other as viewed in the stacking direction. Generally, if two or more raw materials are prepared and an approximately uniform reaction speed is intended in designing a reactor, countercurrent flow is employed in many cases. Meanwhile, employing the above-described configuration realizes cross flow for causing the electrolytic solutions to flow in nearly countercurrent flow. This simulates the concept of the countercurrent flow to achieve a more stabilized reaction speed.

In the present embodiment, in the electrolytic cell stacking structure 13, the cathode-specific electrolytic solution inlet 61 through which a cathode-specific electrolytic solution is supplied and the anode-specific electrolytic solution inlet 63 through which an anode-specific electrolytic solution is supplied are arranged below the first flow path structure 31 and the second flow path structure 32, and the cathode-specific electrolytic solution outlet 62 through which the cathode-specific electrolytic solution is discharged and the anode-specific electrolytic solution outlet 64 through which the anode-specific electrolytic solution after reaction is discharged are arranged above the first flow path structure 31 and the second flow path structure 32.

If gas generated at each of the cathode 21 and the anode 22 stays in the cathode 21 or the anode 22 for failing to be discharged smoothly, an area of contact between an electrolytic solution and the electrode layer 30 is reduced to cause the risk of adverse effect on a reaction speed. However, employing the above-described configuration achieves smooth discharge of the generated gas using buoyant force.

In the present embodiment, the electrolytic cell stacking structure 13 includes the insulating part 60 having elasticity and surrounding a periphery of each of the electrode layer 30, the first flow path structure 31, and the second flow path structure 32.

As a result, the insulating part 60 can be used for adjusting force of interposing the ion exchange membrane 23 with the cathode 21 and the anode 22. The insulating part 60 can also be used for preventing an electrolytic solution from flowing out of the electrolytic cell stacking structure 13.

In the present embodiment, the first flow path structure 31 includes a plurality of the convex anode-side contact parts 42 formed on the surface defining the anode-specific flow path 41 and a plurality of the convex cathode-side contact parts 44 formed on the surface defining the cathode-specific flow path 43, the second flow path structure 32 includes a plurality of the convex anode-side contact parts 52 formed on the surface defining the anode-specific flow path 51 and a plurality of the convex cathode-side contact parts 54 formed on the surface defining the cathode-specific flow path 53, the anode-side contact parts 42 of the first flow path structure 31 and the cathode-side contact parts 54 of the second flow path structure 32 are arranged at positions corresponding to each other in the stacking direction of the electrolytic cell stacking structure 13, and the cathode-side contact parts 44 of the first flow path structure 31 and the anode-side contact parts 52 of the second flow path structure 32 are arranged at positions corresponding to each other in the stacking direction of the electrolytic cell stacking structure 13.

As a result, it is possible to adjust force of interposing the ion exchange membrane 23 with the cathode 21 and the anode 22 using the heights of the anode-side contact parts 42, 52 and those of the cathode-side contact parts 44, 54. This allows the thickness of the electrode layer 30 to be physically controlled with higher accuracy, so that lack of balance in electrical resistance due to thickness differences can be reduced.

In the present embodiment, in each of the first flow path structure 31 and the second flow path structure 32, the anode-side contact parts 42, 52 are arranged in a plurality of rows aligned in a direction intersecting a flow direction of an electrolytic solution, the cathode-side contact parts 44, 54 are arranged in a plurality of rows aligned in a direction intersecting a flow direction of an electrolytic solution, and the anode-side contact parts 42, 52 and the cathode-side contact parts 44, 54 are arranged alternately in such a manner as to avoid overlap therebetween as viewed in the stacking direction of the electrolytic cell stacking structure 13.

As a result, by the presence of the rows of the cathode-side contact parts 44, 54 or those of the anode-side contact parts 42, 52, an electrolytic solution is allowed to develop separately to the right and left several times in a flow direction of the electrolytic solution and to flow uniformly, thereby contributing to uniformity in current density on a reaction surface.

Modifications having configurations different from that of the above-described embodiment will be described next. A structure common or similar to that of the above-described embodiment will be given the same sign and explanation thereof may be omitted.

FIG. 6 is a view showing an electrolytic cell stacking structure 113 of an electrochemical reaction device according to a first modification, as viewed from one side in a stacking direction. The electrolytic cell stacking structure 113 of the first modification has an aspect ratio different from that of the above-described embodiment and is the same as the above-described embodiment in the other configuration. In the first modification, the electrolytic cell stacking structure 113 is short in length in a top-bottom direction (vertical length) and is long in length in a right-left direction (lateral length). Specifically, the electrolytic cell stacking structure 113 is formed into a laterally-long shape as viewed in the stacking direction. As a result, flows of gases generated at the cathode 21 and the anode 22 toward the cathode-specific electrolytic solution outlet 62 and the anode-specific electrolytic solution outlet 64 at higher positions can be brought similar to countercurrent flow, thereby allowing the gases after reaction to be discharged to the outside of the electrolytic cell stacking structure 113 with improved performance.

FIG. 7 is a view showing an electrolytic cell stacking structure 213 of an electrochemical reaction device 3 according to a second modification, as viewed from one side in a stacking direction. FIG. 8 is a view showing a surface on the side of the cathode 21 and a surface on the side of the anode 22 of a first flow path structure 231 according to the second modification.

As shown in FIG. 7 , in an anode-specific flow path 241 of the first flow path structure 231 in the electrolytic cell stacking structure 213 of the second modification, flow path guide walls 250, 251, 252, and 253 are formed upstream from a region where the anode-side contact part 42 and the cathode-side contact part 44 are formed. As viewed in the stacking direction, the flow path guide walls 250 are formed into vertically-long shapes conforming to a flow direction and are arranged in the vicinity of the cathode-specific electrolytic solution inlet 61. The flow path guide walls 251 are tilted in a direction intersecting the flow direction and causing an electrolytic solution to flow even to a position away from the cathode-specific electrolytic solution inlet 61. The flow path guide walls 252 and 253 are formed into substantially circular columnar shapes and are arranged in the vicinity of the anode-side contact part 42 and the cathode-side contact part 44. By the presence of the flow path guide walls 252 and 253, the electrolytic solution is diffused in front of the anode-side contact part 42 and the cathode-side contact part 44 into a direction (right-left direction) intersecting the flow direction further.

In the anode-specific flow path 241 of the first flow path structure 231, flow path guide walls 260, 261, 262, and 263 are formed downstream from the region where the anode-side contact part 42 and the cathode-side contact part 44 are formed. The flow path guide walls 262 and 263 are formed into substantially circular columnar shapes and are arranged separately in the vicinity of the anode-side contact part 42 and the cathode-side contact part 44. By the presence of the flow path guide walls 262 and 263, an electrolytic solution having flowed out of each of the anode-side contact part 42 and the cathode-side contact part 44 is diffused into a direction (right-left direction) intersecting a flow direction. The flow path guide walls 260 are formed into vertically-long shapes conforming to the flow direction and are arranged in the vicinity of the cathode-specific electrolytic solution outlet 62. The flow path guide walls 261 are tilted in a direction intersecting the flow direction and causing an electrolytic solution to flow from a position away from the cathode-specific electrolytic solution outlet 62 toward the cathode-specific electrolytic solution outlet 62.

As shown in FIG. 8 , the first flow path structure 231 and the second flow path structure 232 in the electrochemical reaction device 3 of the second modification are common parts. In the second modification, the first flow path structure 231 has a shape point-symmetrical to the second flow path structure 232 and becomes identical in shape to the second flow path structure 232 by being rotated 180 degrees as viewed in the stacking direction. This allows reduction in parts count required for the electrolytic cell stacking structure, so that manufacturing cost can be reduced.

The flow path guide walls 250 to 253 and the flow path guide walls 260 to 263 of the first flow path structure 231 described above play their roles switched between the upstream side and the downstream side to function as flow path guide walls for guiding a flow of an electrolytic solution in a cathode-specific flow path 242 in the second flow path structure 232. Specifically, the flow path guide walls 260 to 263 are located on the upstream side and the flow path guide walls 250 to 253 are located on the downstream side.

As described above, in the second modification, the cathode-specific flow path 242 and the anode-specific flow path 241 include the flow path guide walls 250 to 253 and the flow path guide walls 260 to 263 for guiding flows of electrolytic solutions.

This allows electrolytic solutions to flow more uniformly, thereby contributing to uniformity in current density on a reaction surface.

In addition, it is appropriately possible to replace the configuration elements in the above-described embodiments with well-known configuration elements without departing from the spirit of the present invention, and the above-described modification examples may be appropriately combined.

-   -   3: Electrochemical reaction device     -   13: Electrolytic cell stacking structure     -   21: Cathode     -   22: Anode     -   23: Ion exchange membrane     -   30: Electrode layer     -   31: First flow path structure     -   32: Second flow path structure     -   41: Anode-specific flow path     -   43: Cathode-specific flow path     -   51: Anode-specific flow path     -   53: Cathode-specific flow path 

What is claimed is:
 1. An electrochemical reaction device for reducing carbon dioxide, the electrochemical reaction device comprising: an electrode layer including a cathode, an ion exchange membrane, and an anode that are stacked in this order; a first flow path structure including an anode-specific flow path as a passage for an electrolytic solution defined by a surface on one side and a cathode-specific flow path as a passage for a cathode-specific electrolytic solution containing dissolved carbon dioxide defined on a surface on an other side; and a second flow path structure including an anode-specific flow path as a passage for an anode-specific electrolytic solution defined by a surface on one side and a cathode-specific flow path as a passage for a cathode-specific electrolytic solution containing dissolved carbon dioxide defined by a surface on an other side, the first flow path structure, the electrode layer, the second flow path structure, and the electrode layer being stacked in this order repeatedly to form an electrolytic cell stacking structure.
 2. The electrochemical reaction device according to claim 1, wherein the electrolytic cell stacking structure includes: a cathode-specific electrolytic solution inlet through which a cathode-specific electrolytic solution is supplied; a cathode-specific electrolytic solution outlet through which the cathode-specific electrolytic solution after reaction is discharged and which is arranged on the opposite side to the cathode-specific electrolytic solution inlet across the first flow path structure and the second flow path structure; an anode-specific electrolytic solution inlet through which an anode-specific electrolytic solution is supplied; and an anode-specific electrolytic solution outlet through which the anode-specific electrolytic solution after reaction is discharged and which is arranged on the opposite side to the anode-specific electrolytic solution inlet across the first flow path structure and the second flow path structure, and the cathode-specific electrolytic solution inlet, the cathode-specific electrolytic solution outlet, the anode-specific electrolytic solution inlet, and the anode-specific electrolytic solution outlet are arranged in such a manner that a line connecting the cathode-specific electrolytic solution inlet to the cathode-specific electrolytic solution outlet and a line connecting the anode-specific electrolytic solution inlet to the anode-specific electrolytic solution outlet intersect each other as viewed in a stacking direction of the electrolytic cell stacking structure.
 3. The electrochemical reaction device according to claim 1, wherein in the electrolytic cell stacking structure, the cathode-specific electrolytic solution inlet through which a cathode-specific electrolytic solution is supplied and the anode-specific electrolytic solution inlet through which an anode-specific electrolytic solution is supplied are arranged below the first flow path structure and the second flow path structure, and the cathode-specific electrolytic solution outlet through which the cathode-specific electrolytic solution after reaction is discharged and the anode-specific electrolytic solution outlet through which the anode-specific electrolytic solution after reaction is discharged are arranged above the first flow path structure and the second flow path structure.
 4. The electrochemical reaction device according to claim 1, wherein the electrolytic cell stacking structure includes an insulating part having elasticity and surrounding a periphery of each of the electrode layer, the first flow path structure, and the second flow path structure.
 5. The electrochemical reaction device according to claim 1, wherein the first flow path structure includes a plurality of convex anode-side contact parts formed on the surface defining the anode-specific flow path and a plurality of convex cathode-side contact parts formed on the surface defining the cathode-specific flow path, the second flow path structure includes a plurality of convex anode-side contact parts formed on the surface defining the anode-specific flow path and a plurality of convex cathode-side contact parts formed on the surface defining the cathode-specific flow path, the anode-side contact parts of the first flow path structure and the cathode-side contact parts of the second flow path structure are arranged at positions corresponding to each other in a stacking direction of the electrolytic cell stacking structure, and the cathode-side contact parts of the first flow path structure and the anode-side contact parts of the second flow path structure are arranged at positions corresponding to each other in the stacking direction of the electrolytic cell stacking structure.
 6. The electrochemical reaction device according to claim 5, wherein in each of the first flow path structure and the second flow path structure, the anode-side contact parts are arranged in a plurality of rows aligned in a direction intersecting a flow direction of an electrolytic solution, the cathode-side contact parts are arranged in a plurality of rows aligned in a direction intersecting a flow direction of an electrolytic solution, and the anode-side contact parts and the cathode-side contact parts are arranged alternately in such a manner as to avoid overlap therebetween as viewed in the stacking direction of the electrolytic cell stacking structure.
 7. The electrochemical reaction device according to claim 1, wherein the first flow path structure has a shape point-symmetrical to the second flow path structure and becomes identical in shape to the second flow path structure by being rotated 180 degrees as viewed in a stacking direction.
 8. The electrochemical reaction device according to claim 1, wherein the cathode-specific flow path and the anode-specific flow path each include a flow path guide wall for guiding a flow of an electrolytic solution. 